Donald Trump railed against Ted Cruz at a Friday morning event, telling an audience of supporters that if the Texas senator were to offer him an endorsement, he would reject it.

"If he gives it, I will not accept it!" Trump exclaimed the day after his acceptance speech at the Republican National Convention.

The Manhattan billionaire told the Cleveland audience that he didn't think Cruz's support was worth anything and urged him to "stay home."

Trump even said he might start a super PAC to try to defeat Cruz if he attempts to run again in 2020.

Trump's ire was trained on Cruz two days after the Texas senator refused to endorse Trump in a primetime speech at the RNC and urged the audience to vote their "conscience" in November's election. Trump said Cruz had "ruined his political career."

"I feel so bad," he joked. "I feel so bad."

The brash billionaire, obviously not using prepared remarks, even floated a conspiracy theory that suggested Cruz's father played a role in the assassination of President John F. Kennedy. It was a suggestion Trump first floated during their heated primary battle — one that caused Cruz to explode in anger at the real-estate mogul at the time.

"There was a picture on the front page of the National Enquirer, which does have credibility," Trump said, falsely contending the magazine is careful not to run false reports because it could be sued.